The present invention relates generally to the field of Magnetic Particle Imaging, and in particular to an open bore coil system enabling electronic steering and rotation of a Field Free Line (FFL) inside a large volume.
Magnetic Particle Imaging (MPI) is an imaging method, which images the concentration of Super Paramagnetic Iron Oxide (SPIO) particles inside a volume of interest. The SPIO nanoparticles can be injected intravenously, or administered orally to the patient/object. It has several potential uses for medical imaging such as angiography, stem cell tracking, and tumor detection.
In the MPI method, the object is placed inside a magnetic field, which includes a field free region (FFR). In this region, the magnetic field strength is very low so that if there are SPIO nanoparticles in this region, their magnetization is not saturated. (i.e. their magnetization can be increased or decreased parallel to the applied magnetic field). This field with the FFR is called the Selection Field (SF), since it is used to select the region in space where the SPIO particles are responsive. The magnetic field with the FFR can be generated using two parallel coils fed with alternating current directions, or using two permanent magnets placed parallel with the same poles looking at each other. The FFR generated with this configuration has an elliptical shape. The magnetization curve of the SPIO particles is non-linear and can be modeled by the Langevin function. This method was first described in Gleich and Weizenecker, “Tomographic imaging using the nonlinear response of magnetic particles,” Nature, vol. 435, 2005.
Because of the super paramagnetic properties of the nanoparticles, their magnetization can be saturated at moderate magnetic field intensity levels. The particles outside the FFR region are saturated because of the higher magnetic field intensity outside the FFR. (i.e. outside the FFR magnetic field strength is high so that the magnetization of the SPIO nanoparticles cannot increase further with increasing magnetic field strength). If a dynamic magnetic field (which is called the Drive Field-DF) is imposed on the SF, the particles inside the FFR respond by aligning their magnetization with the applied field. On the contrary, the particles in the saturated region are not responsive as their magnetization is not affected by the DF. Change in magnetization vector inside the FFR induces a voltage on the receive coil(s). The induced voltage depends linearly on the particle concentration inside the FFR, which can be reconstructed. The coupled signal from the transmit coil(s) to the receive coil(s) is much larger than the received SPIO response. However, as the magnetization response of the particles is non-linear, the received signal includes the harmonics of the excitation frequency. Generally, the fundamental frequency component is filtered out, and the harmonic frequency components are used to reconstruct the particle concentration.
The FFR can be scanned in 3D to obtain 3D SPIO concentration images. This can be done using drive fields in three orthogonal axes. The homogeneous DF can be generated with two parallel coils fed with the same current (Helmholtz coil configuration). Alternatively, a solenoid structure can also be used. The Helmholtz coils can be placed conformally on a tube with circular cross section.
The amplitude and frequency of the DF should be selected to be in safe limits. There are two effects of the applied DF on the biological tissues: peripheral nerve stimulation (PNS) and heating. It was reported that the PNS can be observed at about 15 mT magnetic field intensity in the frequency region commonly used for MPI (25-50 kHz). (E. U. Saritas et al., “Magnetostimulation Limits in Magnetic Particle Imaging,” IEEE Trans. Med. Imag., Vol. 32, no. 9, September 2013). The heating effect depends on the duration and frequency of the dynamic excitation, increasing with both variables. The field of view (FOV) using safe DF amplitude levels is on the order of millimeters. It is possible to steer the FFR inside the object in discrete positions, or continuously with a very low frequency inside safety limits using a homogenous field. This relatively high amplitude and low frequency field is called the Focus Field (FF). A high frequency, lower amplitude DF is applied on top of this field to get the signal from the steered FFR. Extra coils can be used to create the FF. Alternatively, SF or DF coils can be used to generate the FF.
To use MPI method in clinical applications such as angiography, the doctor/medical personnel should be able to physically access the patient to control and direct the process. The vast majority of the MPI systems presented up to date use closed bore scanners, in which the object is placed inside a cylindrical bore and, therefore, not accessible.
The FFR in the MPI method, also called the Field Free Point (FFP), is in the shape of an ellipse with dimensions on the order of millimeters. Since the received signal is induced by the nanoparticles inside this region, the signal level increases with increasing FFR size. However, this contradicts with the image resolution, which decreases with FFR size. The FFP should be scanned in three dimensions, which is a time consuming task especially for relatively large objects such as human body. In a clinical environment, imaging duration should be kept as small as possible to enable real time imaging and to prevent image distortions caused by patient movement.
In summary, even though the MPI field is rapidly developing, the prototypes or products up to date are limited to closed bore scanners with a small FOV size only suitable for small animal experiments. To use the MPI method in the clinic, a fast open bore scanner with a large FOV size is required. In addition, the applied magnetic field levels should be kept below safety limits.
Thus, there is a need for a coil system, which can provide Selection, Focus and Drive fields to meet these clinical requirements.